I remember reading a rumor somewhere that Orson Welles had played a hand in directing, but this was debunked. I personally think it is not just Carol Reed's best film (out of the ones I've seen) but also one of the best, if not the best, British film ever made

What we know is that he contributed dialogues and shaped his character, not that he actually directed some of his scenes.

As I said above, the theory that he directed some scenes has been largely debunked and is quite disrespectful to Reed. That Welles contributed dialogue and shaped his character is no different from what he seems to have done in any film that he actually cared about and wasn't just doing for the money.